1. Field of the Invention
This invention relates to a ferroelectric liquid crystal material. More particularly, it relates to a ferroelectric liquid crystal composition comprising smectic liquid crystal compound(s) and optically active compounds and having a spontaneous polarization value of 20 nC cm.sup.-2 or less and nevertheless having quick response properties, and to a light-switching element using the above ferroelectric liquid crystal composition.
2. Description of the Related Art
Liquid crystal compounds have been broadly used as materials for display elements, but most of such liquid crystal display elements have a TN display mode, and as liquid crystal materials, those having a nematic phase have been used.
TN mode display devices have characteristics which result in eyes are not being strained due to a non-emissive mode, and also the power consumption is very small. However, they have the drawbacks that the response rate is low and, depending upon the angle of view the display is not always seen.
However, the mode has recently been considered for making use of its specific feature for flat display, and in particular, quick response properties and a broad angle of view have been required therefor.
In order to provide such requirements, improvement in liquid crystal materials has been attempted. However, as compared with emissive mode displays such an electro-luminescent display, etc., TN the mode display has differs considerably from the emissive mode displays in the aspects of response time and breadth of angle of view.
In order to make use of the specific features of a non-emissive mode and a small power consumption and yet secure response properties matching the emissive mode display, development of a novel liquid crystal display mode in place of the TN display mode is indispensable.
As one of such attempts, a display mode utilizing the light switching phenomenon of ferroelectric liquid crystals was proposed by N. A. Clark and S. T. Lagewall (see Applied Physics. Lett. 36, 899, 1980).
As to ferroelectric liquid crystals, their existence was announced by R. B. Meyer, et al. for the first time (see J. Phys. 36, 69, 1975), and in the aspect of liquid crystalline structure, the crystals belong to the chiral smectic C phase, chiral smectic I phase, chiral smectic F phase, chiral smectic G phase and chiral smectic H phase (hereinafter abbreviated to SC*, SF*, SI*, SG* and SF*, respectively).
In chiral smectic phases, molecules form a layer and are tilted against the layer plane and their helical axis is vertical to the layer plane.
In the chiral smectic phase, since spontaneous polarization is formed therein, when a direct current electric field is oriented parallel to the layer, molecules are reversed around their helical axis as a rotating axis, depending upon their polarity. Display elements using ferroelectric liquid crystals utilize the switching phenomenon at the time of this reversal.
Among the chiral smectic phases, the SC* phase has now been particularly noted.
As the display mode utilizing the switching phenomenon of the SC* phase, two modes are considered. One of the modes is a birefringence mode using two polarizers, and the other is a guest-host mode utilizing a dichroic dyestuff.
The display mode has specific features such as
(1) a quick response rate,
(2) memory properties,
(3) a small dependency upon angle of view, etc., and has a possibility of high density display; hence the mode is very attractive for display elements. However, in the present situation, the display mode has a number of problems to be solved.
Recently, the relationship between the value of spontaneous polarization and memory properties and the relationship between the value of spontaneous polarization and switching of SC* phase has been discussed.
A very important problem has been observed that when a ferroelectric liquid crystal material having a spontaneous polarization value of 20 nC cm.sup.-2 or more is placed in a cell provided with an insulator layer, multiplex drive is impossible due to the extinction of memory properties.
With regard to such abnormal behavior, it has been reported that in the case of a ferroelectric liquid crystal material having a spontaneous polarization value of 20 nC cm.sup.-2 or more, a reverse voltage generated by an electric field (polarized field or ionized field) generated in the reverse direction to the dipole direction of liquid crystal molecules inside the cell is higher than the threshold voltage; hence switching effected by impressing a pulse voltage is impossible (for example, see Masahito Ishikawa, et.al: the 14th Liquid Crystal Symposium, Japan, p. 100-101 (1988), or J. Dijon, et.al: SID 88 DIGEST, p. 246-249 (1988)).
In general, in order to avoid the above abnormal behavior when a polyimide membrane is used as an insulator layer, the spontaneous polarization value of ferroelectric liquid crystal materials has been said to be necessary to be 20 nC cm.sup.-2 or less, preferably 15 nC cm.sup.-2 or less. Further, practically, in the case where the above materials are used for a display element of 640.times.400 lines, the following properties are particularly required:
(1) exhibiting a SC* phase within a broad temperature range including room temperature,
(2) exhibiting a short response time (150.mu.sec or less) and
(3) exhibiting superior aligning properties.
At present, however, ferroelectric liquid crystal materials (liquid crystal compositions) having a spontaneous polarization value of 20 nC cm.sup.-2 or less and satisfying all of the above-mentioned conditions have not been obtained yet.
For example, Japanese patent application laid-open No. Sho 61-291679 and PCT international application laid-open No. WO 86/06401 (pamphlet) disclose a ferroelectric liquid crystal composition obtained by mixing an achiral 5-alkyl-2-(4-alkoxyphenyl)pyrimidine having a SC phase with an optically active compound, and also disclose that the composition has a spontaneous polarization value of 20 nC cm.sup.-2 or less and exhibits a Sc* phase within a broad temperature range including room temperature.
Further, the former reference discloses that since the pyrimidine derivative has a very low viscosity, if it is used as a base Sc mixture in ferroelectric liquid crystal compositions, it is possible to shorten the response time of a light-switching element.
Further, the above Japanese patent application laid-open No. Sho 61-291679 also discloses that a ferroelectric liquid crystal material consisting of a 5-alkyl-2-(4'-alkylbiphenyl-4)pyrimidine, the above 5-alkyl-2-(4-alkoxyphenyl)pyrimidine and an optically-active compound also exhibits a Sc* phase within a broad temperature range including room temperature and also is effective for improving response properties.
However, the ferroelectric liquid crystal composition disclosed in the above Japanese patent application laid-open No. Sho 61-291679 and PCT international patent application laid-open No. WO 86/06401 has a spontaneous polarization value and a temperature range of the Sc* phase each satisfying the above requirements, but its response time it is still long (300 to 600.mu.sec), that is, is not regarded as practical; hence a further improvement in response properties has been earnestly desired.
Further, U.S. Pat. No. 4,882,083 discloses a ferroelectric liquid crystal composition consisting of an achiral pyrimidine compound having a Sc phase and an optically-active compound expressed by formula (III) of the present invention and discloses that this composition exhibits a Sc* phase within a broad temperature range including room temperature and yet exhibits a very quick response rate. For example, a ferroelectric liquid crystal composition described in Example 5 of the above-noted U.S. Patent exhibits Sc* a phase within a broad temperature range of 5.degree. to 52.degree. C. and a response time of 100.mu.sec, that is, satisfies it the above-mentioned requirements; thus it appears that the ferroelectric liquid crystal composition is very practical as compared with that described in the above Japanese patent application laid-open No. Sho 61-291679 or PCT international patent application laid-open No. WO 86/06401.
However, the ferroelectric liquid crystal composition described in the above U.S. Pat. No. 4,882,083 still raises a serious problem in its practical use. Namely, the composition has no cholesteric phase; thus, according to the aligning technique currently employed for TN liquid crystal materials, no uniform alignment has been obtained.
At present, as a method for aligning ferroelectric liquid crystal materials, three types of a shearing method, a temperature gradient method and a surface-treatment method have been attempted. The shearing method is directed to effecting alignment by applying a shearing stress in smectic A phase. The temperature gradient method is a similar method to an epitaxial liquid crystal growth method noting that the smectic phase can be regarded as a one-dimentional crystal. The surface treatment method, which has been practically employed for aligning TN liquid crystal materials, is a method of coating a polymer membrane such as a polyimide membrane onto the substrate of a cell and subjecting the surface to a rubbing treatment to align liquid crystal molecules. In the aspect of commercial production of liquid crystal display elements, it is preferred to align liquid crystal molecules according to the surface treatment method.
The phase transition types of ferroelectric liquid crystal materials include the following 4 types:
a) Iso phase.fwdarw.Sc* phase, PA0 b) Iso phase.fwdarw.N* phase.fwdarw.Sc* phase, PA0 c) Iso phase.fwdarw.S.sub.A phase.fwdarw.Sc* phase and PA0 d) Iso phase.fwdarw.N* phase.fwdarw.S.sub.A phase.fwdarw.Sc* phase
wherein Iso phase, N* phase and S.sub.A phase represent isotropic liquid phase, cholesteric phase and smectic A phase, respectively.
Among these, the type which can utilize the current aligning technique (surface treatment method) as it is, is that having a phase transition series expressed by the type d) (see Japanese patent application laid-open No. Sho 61-250086 for example). Thus, a ferroelectric liquid crystal material having the phase transition type expressed by d) has been particularly desired.
The ferroelectric liquid crystal composition disclosed in the above U.S. Pat. No. 4,882,083 has no cholesteric phase, and in order to uniformly align the composition, it is necessary to employ the shearing method or the temperature gradient method, but a long time is required for the alignment, it being impossible to effect the alignment easily Further, since the current aligning technique cannot be utilized as it is, new equipment investment is required. Thus the above composition cannot be said to be practical at all.
As apparent from the foregoing, the ferroelectric liquid crystal compositons known so far are still difficult to be regarded as practical, and hence a further improvement in the characteristics has been earnestly desired.